@fedify/backfill: ActivityPub backfill for Fedify

This package is available since Fedify 2.3.0.

This package provides ActivityPub conversation backfill support for the Fedify ecosystem. It can retrieve post-like objects from a seed object's context collection, following the direct FEP-f228 path where the context dereferences to a Collection, OrderedCollection, CollectionPage, or OrderedCollectionPage. It can also use an opt-in reply-tree strategy to walk inReplyTo ancestors and replies descendants when context collections are unavailable or incomplete.

Installation

deno add jsr:@fedify/backfill
npm  add     @fedify/backfill
pnpm add     @fedify/backfill
yarn add     @fedify/backfill
bun  add     @fedify/backfill

Usage

The backfill() function accepts a backfill context, a seed object, and traversal options:

import { backfill } from "@fedify/backfill";
import { lookupObject } from "@fedify/vocab";

const documentLoader = (iri: URL, options?: { signal?: AbortSignal }) =>
  lookupObject(iri, { signal: options?.signal });

for await (
  const item of backfill({ documentLoader }, note, {
    maxItems: 20,
    maxRequests: 50,
  })
) {
  console.log(item.id?.href);
}

The seed object itself is not yielded. If it appears in the discovered collection, it is skipped by ID.

Configured strategies run in order. They share maxItems, maxRequests, abort state, and object ID deduplication; if two strategies discover the same object, the first strategy keeps its BackfillItem metadata.

By default, backfill() uses the context-auto strategy. In this mode, collection items are treated as backfillable objects by default. If an item is recognized as a supported Create activity, backfill() extracts the activity's object instead.

To accept only post-like objects directly contained in the context collection, use the context-objects strategy:

for await (
  const item of backfill({ documentLoader }, note, {
    strategies: ["context-objects"],
  })
) {
  console.log(item.object);
}

To read only FEP-f228 activity collections, enable the context-activities strategy:

for await (
  const item of backfill({ documentLoader }, note, {
    strategies: ["context-activities"],
  })
) {
  console.log(item.object);
}

The context-activities strategy currently supports Create activities and yields the activity's object, not the activity itself.

To combine the FEP-f228 context collection path with traditional reply-tree crawling, add the reply-tree strategy after context-auto:

for await (
  const item of backfill({ documentLoader }, note, {
    strategies: ["context-auto", "reply-tree"],
    maxDepth: 4,
  })
) {
  console.log(item.origin, item.depth, item.object);
}

The reply-tree strategy walks inReplyTo ancestors and replies descendants. It yields discovered post-like objects only; it does not extract objects from Activity wrappers. Immediate parents and direct replies have depth 1, their next-level parents or replies have depth 2, and so on. Reply-tree traversal defaults to a maximum depth of 10; set maxDepth to use a different limit.

Traversal controls

All configured strategies share the same traversal controls:

• maxItems limits the number of yielded objects. Skipped duplicates do not count.
• maxRequests limits calls to documentLoader. Embedded objects and collections do not count.
• maxDepth limits reply-tree traversal and defaults to 10. It does not limit context collection items.
• interval adds a delay between loader requests. Its callback receives the zero-based request index.
• signal cancels traversal and is forwarded to documentLoader.

An interval string requires the global Temporal API or a polyfill. Temporal.DurationLike objects work without the global API.

If the seed has no context, or its context resolves to a non-collection, context strategies yield nothing. Loader failures are skipped unless traversal is aborted.

Dereferenced documents are cached in memory for one backfill() traversal. Applications that need persistent or shared caching can provide it through the documentLoader.